Most bumps and blotches on a newborn baby are harmless and clear up by themselves. By far the most common skin problem in infants is diaper rash. Diaper rash is an irritation of the skin caused by dampness, urine, or feces. Most babies who wear diapers will have some type of diaper rash. However, there are other skin disorders that can cause rashes. These are usually not serious unless accompanied by other symptoms.
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2

Not from histamine:
Despite your age, you need to get your cardiac status evaluated before you exert heavily again. This may be a warning sign of something serious and potentially life-threatening.
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3

Several causes:
If the vault (the central "pit" area) is involved, it's usually caused by allergy to a deodorant or antiperspirant, or some component of it. If the outer rim of the armpit is involved and the vault is spared, it's usually caused by an article of clothing or detergent or fabric softener. Other causes include yeast and fungus infections, and there are other, more obscure causes.
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4

Variety of Things:
This could be related to clothing you have worn, a hair follicle/ingrown hair, an infection in a sweat gland, a reaction to something that touched that portion of your skin, an infection in your skin, or something else. It is difficult to tell without more of a story/seeing the rash. Antiperspirants are the leading cause of armpit problems in the us.
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8

Allergic reaction:
Personal care items like deodorants or body creams can some times cause allergic reactions with blistering. If it is only on one side and painful consider a viral infection.
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9

ARMPIT RASH:
WHY UNDER ONE ARM I DON'T KNOW.
IF ITS UNDER THE ARM THAT DOES ALL THE WORK, LIKE WRITING, LIFTING, EATING, ETC., IT MAY BE A CONTACT DERMATITIS FROM SOAP, DETERGENT, STUFF YOU MAY BE PUTTING ON WITH YOUR DOMINANT HAND.
BE A DETECTIVE AND IF IT DOESNT GET BETTER, SEE DERMATOLOGIST.
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